This application is based upon and claims the benefit of priority from prior Japanese Patent Application No. 2002-118233, filed on Apr. 19, 2002, the entire contents of which are incorporated herein by reference.
The present invention relates to an amplification circuit, and more specifically relates to an amplification circuit provided in an optical communication apparatus.
In recent years, electronic devices such as portable telephones, and portable terminals such as personal digital assistants (PDAs) have been provided with infrared data communication functions. Computers have been provided with optical communication apparatuses for sending and receiving data over optical fiber. These optical communication apparatuses include an amplification circuit as an optical receiver amplifier (amp).
As shown in FIG. 1, a first example of a conventional optical receiver amp 10 is connected to a photodiode PD. The photodiode PD generates a reception current IPD corresponding to the amount of light received, and the optical receiver amp 10 generates a reception signal RX in accordance with the reception current IPD. The optical receiver amp 10 includes a pre-amp 11, a main amp 12, and a comparator 13.
The pre-amp 11 includes a resistor R1 connected between an operating power supply VREG and the photodiode PD, and a diode D1 connected in parallel with the resistor R1. The diode D1 is connected to the resistor R1 in the forward direction relative to the current flowing through the resistor R1. The pre-amp 11 converts the reception current IPD to a voltage signal VFM. The amount of change xcex94VFM of the voltage signal VFM relative to the amount of change xcex94IPD of the reception current IPD is represented by the expression xcex94VFM=xcex94IPDxc3x97R1. The operating power supply VREG may supplied through a power supply filter provided internally or externally to an integrated circuit (IC) built into the receiver amp 10, or may be supplied from a constant-voltage regulated power supply.
The main amp 12 amplifies the voltage signal VFM, and generates an amplification signal VA. The comparator 13 converts the amplification signal VA of the main amp 12 to a digital reception signal RX using a threshold voltage VTH.
An optical receiver amp 20 of a second conventional example is a differential-type amp, as shown in FIG. 2, and includes a pre-amp 21, a buffer circuit 22, a bandpass filter 23, a main amp 24, a comparator 25, and a DC light-canceling circuit 26.
The pre-amp 21 includes two resistors R2 and R3, four transistors Q1 through Q4, and two current sources 27 and 28, and generates a differential output signal. The resistor R2, transistor Q1 and current source 27 are connected in series between an operating power supply VREG and a low-potential power supply, and the resistor R3, transistor Q2 and current source 28 are connected in series between the operating power supply VREG and the low-potential power supply. A bias voltage VB is supplied to the bases of the transistors Q1 and Q2. A photodiode PD is connected at a node between the transistor Q1 and the current source 27.
The emitter of the transistor Q3 is connected between the resistor R2 and the transistor Q1, and the emitter of the transistor Q4 is connected at the node between the resistor R3 and the transistor Q2. A high-potential power supply Vcc supplies power to the collectors of the transistors Q3 and Q4, and a clamp voltage Vc is applied to the bases of the transistors Q3 and Q4.
The pre-amp 21 generates a main voltage signal VFM at a node between the resistor R2 and the transistor Q1, and generates a reference voltage signal VFP at a node between the resistor R3 and the transistor Q2.
When a reception current is not generated by the photodiode PD, the clamp voltage Vc, and base-emitter voltage VBE of the transistors Q3 and Q4, and the voltage signal VFM have the relationship VCxe2x88x92VBE greater than VFM, and the transistors Q3 and Q4 are turned OFF. The transistors Q3 and Q4 are turned ON when a relatively large input current is supplied, and the voltage signals VFM and VFP are clamped at predetermined voltages.
The amount of change xcex94VFM in the voltage signal VFM relative to the amount of change xcex94IPD in the reception current IPD is represented by the expression xcex94VFM=xcex94IPDxc3x97R2. The buffer circuit 22, the bandpass filter 23, and the main amp 24 amplify the differential voltage xcex94VF (xcex94VPFxe2x88x92xcex94VFM) of the differential output signal of the pre-amp 21, and generate an amplified differential output signal. The comparator 25 converts the differential output signal from the main amp 24 to a digital reception signal RX.
The DC light-canceling circuit 26 cancels the direct current component (DC component) included in the reception current IPD flowing through the photodiode PD. The DC component is generated by background DC light, such as sunlight and the like, and includes a frequency component lower than the predetermined frequency band including the communication frequency. The DC light-canceling circuit 26 provides feedback for the current canceling the canceled DC component, which is included in the voltage signals VFM and VFP, to the input of the pre-amp 21.
When light input to the photodiode PD includes light components other than communication light, the total gain of the optical receiver amps 10 and 20 is reduced, and the signal-to-noise (S/N) ratio of the optical receiver amps must be increased. Generally, an auto gain control (AGC) circuit is used as a means of reducing the gain. A more effective method of simply reducing the gain is to adjust the resistance value of the resistor R1 (resistors R2 and R3 of pre-amp 21) of the pre-amp 11 via an AGC circuit. However, when the resistance values of these resistors R1 through R3 are adjusted, the bias voltage fluctuates in conjunction with the variation in the resistance values. Then, a suitable bias voltage is not supplied to later-stage amps. Particularly when the optical receiver amps 10 and 20 are operated by a low voltage power supply, there is not enough margin in the bias level between amps.
When the resistance values of the resistors R1, R2, and R3 are adjusted by an AGC circuit, the amount of attenuation (amount of change in the gain) is approximately xe2x88x9230 dBxcexa9. When the optical input signal is relatively large, the gain must be reduced, which naturally requires another circuit. However, adding this circuit increases the circuit area of the optical receiver amp.
In a first aspect of the present invention, an amplification circuit for receiving an input current is provided. The amplification circuit includes a first amplifier including a current-to-voltage conversion resistor for generating a first voltage signal corresponding to the input current. A second amplifier is connected to the first amplifier to amplify the first voltage signal and generating a second voltage signal. A first gain control circuit is connected to the first and second amplifiers to generate a first gain control signal based on the second voltage signal and adjusting the resistance value of the current-to-voltage conversion resistor in accordance with the first gain control signal. A bias control circuit is connected to the first amplifier and the first gain control circuit to generate a bias control signal based on the gain control signal and adjust the bias current at the output of the first amplifier in accordance with the bias control signal.
In a second aspect of the present invention, an optical communication apparatus for receiving signal light is provided. The optical communication apparatus includes a first photoreceptor element for receiving the signal light and generating a first reception current. A first amplifier is connected to the first photoreceptor element and includes a current-to-voltage conversion resistor. The first amplifier generates a first voltage signal corresponding to the first reception current. A second amplifier is connected to the first amplifier to amplify the first voltage signal and generating a second voltage signal. A first gain control circuit is connected to the first and second amplifiers to generate a first gain control signal based on the second voltage signal and adjust the resistance value of the current-to-voltage conversion resistor in accordance with the first gain control signal. A bias control circuit is connected to the first amplifier and the first gain control circuit to generate a bias control signal based on the gain control signal and adjusting the bias current at the output of the first amplifier in accordance with the bias control signal.
In a third aspect of the present invention, there is provided an optical communication apparatus which includes a transmission circuit for generating a transmission current in accordance with a transmission signal, a light-emitting element for generating transmission signal light in accordance with the transmission current and generating a signal light detection signal, a photoreceptor element for receiving a signal light and generating a reception current, a reception circuit connected to the photoreceptor element for generating a reception signal corresponding to the reception current. A switching circuit is connected to the light-emitting element, the transmission circuit and the reception circuit, to connect the light-emitting element and the transmission circuit in a transmission mode and connect the light-emitting element and the reception circuit in a reception mode. The reception circuit includes a first amplifier including a current-to-voltage conversion resistor for generating a first voltage signal corresponding to the reception current. A second amplifier is connected to the first amplifier to amplify the first voltage signal and generating a second voltage signal. A first gain control circuit is connected to the first and second amplifiers to generate a first gain control signal based on the second voltage signal and adjusting the resistance value of the current-to-voltage conversion resistor in accordance with the first gain control signal. A bias control circuit is connected to the first amplifier and the first gain control circuit to generate a bias control signal based on the gain control signal and adjust the current at the output of the first amplifier in accordance with the bias control signal. A second gain control circuit is connected to the light-emitting element and the first amplifier to generate a second gain control signal based on the detection signal and adjust the gain of the first amplifier in accordance with the second gain control signal.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrated by way of examples of the principles of the invention.